For many years the partial vacuum created in the inlet manifold of a petrol engine has been utilized to exhaust the reservoir of a vacuum brake servo, thereby to provide power assistance for the vehicle brakes. Such a system is simple and extremely reliable.
The vacuum available from the inlet manifold of a petrol engine may however be insufficient to meet brake servo demand in certain conditions of use. Furthermore the vacuum source may also be required for operation of other devices such as exhaust gas recirculation (EGR) valves.
Diesel engines have an unthrottled air supply and thus the partial vacuum in the inlet manifold is only marginally below atmospheric pressure; consequently a useful vacuum source is not available. Accordingly mechanically operated vacuum pumps have been proposed for cars and light trucks equipped with a conventional vacuum brake servo. Such pumps may driven from the engine camshaft by, for example, an axially aligned drive coupling, a camshaft follower or a belt driven pulley arrangement.
In instances where the pump is driven by a drive coupling, it is highly desirable to lubricate engagement faces of the drive coupling so that the drive coupling does not wear excessively. One manner in which the drive coupling can be lubricated is by positioning the outlet to the vacuum pump such that oil expelled through the pump outlet impinges upon the drive coupling. The oil used for lubricating the drive coupling is thus oil that has previously been admitted into a vacuum generating chamber of the pump for the purpose of sealing clearances between moving parts of the pump.
The position in which the pump is located and/or the manner in which the pump is mounted to the engine may prevent the pump outlet being provided in a position where it is able to direct oil onto the drive coupling. Alternatively, the pump may be of a type which does not need oil to be introduced into the pump chamber in order to seal clearances, and thus there is no oil expelled through the outlet which could be used to lubricate the drive coupling.